This invention relates to a method and apparatus for removing liquid from a liquid-solids mixture. The invention is particularly applicable to slurries containing a polymerization catalyst solid product.
A continual problem in the field of polymerization catalyst production is drying of the catalyst product. Typical processes produce a slurry comprising the desired solid catalyst product in admixture with a solvent or solvents. Removal of the solvent is therefore necessary.
One prior technique involves replacing a typical process solvent such as toluene with a lighter, more volatile solvent in a washing tank. The object is to select a lighter solvent, such as butane, which can vaporize, unlike toluene, at "positive" pressures greater than atmospheric pressure. Removing the lighter solvent at a positive pressure in this manner is intended to minimize the risk of contamination and possible deactivation of the catalyst which can result from oxygen and/or water vapor entering the system from the surrounding environment. However, this desired reduction in contamination is to a large extent defeated by the solvent exchange process itself which requires additional equipment and process steps to thereby increase the chance of catalyst contamination. In addition, the requirement for such additional equipment, solvent and process steps makes this drying technique undesirably expensive. Finally, optimum drying is not usually achievable at the pressure employed.